Touch sensors are transparent or opaque input devices for computers and other electronic systems. As the name suggests, touch sensors are activated by touch, either from a user's finger, or a stylus or some other device. Transparent touch sensors, and specifically touchscreens, are generally placed over display devices, such as cathode ray tube (CRT) monitors and liquid crystal displays, to create touch display systems. These systems are increasingly used in commercial applications such as restaurant order entry systems, industrial process control applications, interactive museum exhibits, public information kiosks, pagers, cellular phones, personal digital assistants, and video games.
The dominant touch technologies presently in use are resistive, capacitive, infrared, and acoustic technologies. Touchscreens incorporating these technologies have delivered high standards of performance at competitive prices. All are transparent devices that respond to a touch by transmitting the touch position coordinates to a host computer. An important aspect of touchscreen performance is a close correspondence between true and measured touch positions at all locations within a touch sensitive area located on the touch sensor (i.e., the touch region).
FIGS. 1a and 1b illustrate a typical resistive touchscreen 20 in which voltage gradients have been sequentially applied to the surface of the touchscreen in x- and y-directions by electrically exciting subsets of electrodes located on the touchscreen. Resistive touchscreens of this type are typically known as 5-wire touchscreens (in contrast to 4-wire touchscreens in which x- and y-gradients are applied to distinct layers). When the resistive touchscreen is touched, the x- and y-coordinates of the touch location can be determined based on a measured voltage potential. As illustrated in FIGS. 1a and 1b, the voltage gradients applied to a touchscreen can be represented as equipotential lines (i.e., lines along which the voltage is constant). These equipotential lines (shown as dashed lines) generally extend between the top and bottom sides of the touchscreen (FIG. 1a) to provide a means for determining the x-coordinate of the touch location, and generally extend between the left and right sides of the touchscreen (FIG. 1b) to provide a means for determining the y-coordinate of the touch location. Also shown in FIGS. 1a and 1b is the direction (solid lines) of associated current flow as a consequence of the spatially varying potentials in the touchscreen.
Resistive touchscreen data are simplest to process when the relationship of measured potentials on the touchscreen is related to Cartesian coordinates in a simple and well-understood manner. In the mathematically simplest case, the measured potentials in the x- and y-directions are linearly related to the coordinates at the point touched. In this manner, the space defined by the voltage gradients can be directly mapped to the Cartesian space. For ideal linear touchscreen performance, the equipotential lines, i.e., lines along which the voltage is constant, must be perfectly straight lines, as shown in FIGS. 1a and 1b. Though many schemes have been proposed, commercial implementations of resistive touchscreens have generally employed linear relationships between measured potentials and spatial coordinates. For example, touchscreens that utilize complex peripheral discrete electrode patterns, such as those found in Elo TouchSystems' AccuTouch™ products and disclosed in U.S. Pat. No. 5,045,644, may be used to generally provide such linearity (at least at a predefined distance from the peripheral electrode patterns).
In simpler touchscreen designs, the measured potentials in the x- and y-directions may be not be linearly related, i.e., the equipotential lines are not straight. One of the simplest designs is that shown in the computer simulated touchscreen 10 of FIGS. 2a and 2b. This configuration consists of four electrodes 12 in the form of quarter-circles located at the corners of a rectangular, uniformly conducting touchscreen 10. FIG. 2a illustrates the equipotentials 14 when a unit potential difference is applied in the x-direction between the left and right electrode pairs, resulting in equipotential lines 14 that generally run in the y-direction. FIG. 2b illustrates the equipotentials 14 when the potential difference is maintained in the y-direction between the top and bottom electrode pairs, resulting in equipotential lines 14 that generally run in the x-direction. The term “generally” is used to stress that the uniform fields or equipotentials need not necessarily run parallel to the x- or y-axes. There is distortion (i.e., equipotentials are not evenly spaced nor are they parallel to the x- and y-axes), since the electrodes 12 are not designed to produce uniform fields.
Although the equipotential lines 14 illustrated in FIGS. 2a and 2b are non-linear, the space defined by the equipotential lines can be mapped to the Cartesian space in many cases. In this manner, every point on that touchscreen surface must have a unique value for the pair of potentials at that point. In topology, this uniqueness is expressed in terms of “topological equivalence.” Two surfaces are topologically equivalent when all points on one surface can be mapped to unique points on the other surface.
When the voltage gradient is applied in the x-direction, let the voltage potential at any given touch location (x,y) be represented by the 2-D function v(x,y). Likewise when the voltage gradient is applied in the y-direction, let the 2-D potential function be represented by w(x,y). Note that in general, both the potential functions v(x,y) and w(x,y) are functions of both x and y. Because of this interdependence of potential upon both x and y, a single potential measurement cannot uniquely specify either x or y. Consequently, it is not possible to measure x independent of y, and y independent of x. The pair of potentials [v(x,y),w(x,y)], however, uniquely transforms to a point P(x,y) in the Cartesian space under certain conditions. Thus, with this uniqueness, operations can be found that will map the pair [v(x,y),w(x,y)] to a unique point P(x,y) in the Cartesian space.
The electrode configuration illustrated in FIGS. 2a and 2b would at first glance appear to be satisfactory when used under the umbrella of topological mapping concepts. Compared to linear touchscreens, non-linear touchscreens may be more economical to produce and demand less energizing power. Although the computational demands upon the processor may increase in order to uniquely map the measured potentials onto the Cartesian surface, the non-linear solution becomes increasingly attractive as the cost and performance of electronic processors improves.
As long as there is topological equivalence between the equipotential surface and the Cartesian surface, the use of non-linear architectures remains a viable solution. It can be seen from FIGS. 2a and 2b that an equipotential pair can be transformed to unique Cartesian coordinates over almost the entire area of the resistive surface. In the vicinity of the electrodes 12, however, there remains a problem of uniqueness. If a circular electrode is polarized in the x-direction, equipotentials near the circle are essentially circular and if polarized in the y-direction, these equipotentials are also essentially circular. Thus, the two equipotentials will be substantially collinear (i.e., parallel to each other). Because it is the intersection of these two equipotentials at a single point that uniquely determines the touch position, each pair of equipotentials created at a particular point in this small region near the electrodes 12 will not in practice uniquely transform to a Cartesian coordinate. This results in regions around the electrodes that are not suitable as a touch region—an undesirable effect in today's highly competitive touchscreen market. This will be referred to as the “poor-crossing” problem, and the regions where this occurs as the “poor-crossing” regions.
This poor-crossing problem is exacerbated by noise or uncertainty in the measurement of the voltage potentials, which tends to blur the intersection points of equipotential lines, such that a unique position cannot be determined within the poor-crossing region. Thus, the poor-crossing problem may exist even if the equipotential pair intersects—albeit at shallow angles. For example, FIG. 3 illustrates the intersection of two equipotentials, v and w formed from alternately biasing the touchscreen in two directions. The scale is magnified, such that the equipotentials are approximately linear over the region shown. The variances in the equipotentials, δv and δw, due to noise and voltage-measurement uncertainty are also illustrated. Thus, the apparent touch position will have a displacement from the true touch position within this region. If v is measured as v±δv, and w as w±δw, then the possible errors in touch position are given by the four radial vectors from the v-w intersection. There are two unique magnitudes for the radial error depending upon the orientation of the equipotentials and signs of the voltage errors. The radial error can be calculated as:
      Ec    =                                                δ            ⁢                                                  ⁢                          v              ·                                                ∇                  →                                ⁢                w                                              ±                      δ            ⁢                                                  ⁢                          w              ·                                                ∇                  →                                ⁢                v                                                                                                                    ∇              →                        ⁢            v                    ×                                    ∇              →                        ⁢            w                                        ,where the (negative of the) electric fields for the horizontal and vertical biases are written as
                    ∇        →            ⁢      v        =                                                      ∂              v                                      ∂              x                                ⁢                      i            ^                          +                                            ∂              v                                      ∂              y                                ⁢                      j            ^                    ⁢                                          ⁢          and          ⁢                                          ⁢                                    ∇              →                        ⁢            w                              =                                                  ∂              w                                      ∂              x                                ⁢                      i            ^                          +                                            ∂              w                                      ∂              y                                ⁢                      j            ^                                ,with î and ĵ as unit vectors in the x and y directions, respectively. The maximum radial error can be selected according to the numerator for fixed δv and δw. This corresponds to the correlated error Ec. Note that when the error voltages, δv and δw, are consistently correlated, the radial error is large (as given by the formula) in two diagonal quadrants and smaller in the other two quadrants of a simple rectangular touchscreen. The relative correlated error at any point on the touchscreen can be evaluated by taking δv=δw=ΔV and determining Ec/ΔV, where the sign of the numerator is always chosen to maximize the result. The above equation clearly shows that, if the two equipotentials intersect at a shallow angle or if a potential gradient is small, the error is large, which is the case for the poor-crossing regions in FIGS. 2a and 2b. A similar calculation can also be performed where the error voltages, δv and δw, are uncorrelated or partially correlated. Such cases generally tend to show the same problems of uniqueness in the poor-crossing regions.
Various electrode configurations that are not circular, such as L configurations, can be visualized to decrease the size of these poor-crossing regions, but the problem still exists. One solution that has been successful to a large extent involves placing bands of intermediate resistivity (i.e., forming a frame) between the low resistivity electrodes themselves and surrounding the high resistivity touch region. In this manner, an effective way to transition from perfectly conductive electrodes situated at the four corners of the touchscreen and the touch region is achieved.
For example, the touchscreen 20 illustrated in FIGS. 1a and 1b comprises a resistive substrate 22 forming a touch region 24, four electrodes 26 disposed in the respective corners of the substrate 22, and an intermediate resistive band 28 (i.e., frame) formed between the touch region 24 and electrodes 26. In the case where the resistance of the touchscreen 24 is very large compared to the resistance of the resistive band 28, the equipotentials are linear as shown. In contrast, in a case where the resistance of the touchscreen 24 is not very large compared to the resistance of the resistive band 28, the equipotentials are non-linear. When designing the touchscreen 20, the ratio of the frame resistance over the substrate resistance, referred herein as the resistance ratio β, must be balanced between two countervailing factors—low-power consumption and linearity. This resistance ratio β will play an important role in the discussion that follows. Before proceeding, let us define it carefully. First of all, β is shorthand notation for either βx or βy, which refer to the resistance ratio for x- and y-excitations respectively. Let Rleft, Rright, Rtop, and Rbottom, be the resistances of band segments 28a, 28b, 28c, and 28d respectively. These are the resistances of the band segments between the electrodes 26 under the hypothetical assumption that the band segments 28a, 28b, 28c, and 28d are electrically isolated from the touch region 24 and from each other. We define the band resistance in the x-direction as rx=Rtop∥Rbottom=Rtop*Rbottom/(Rtop+Rbottom), and the band resistance in the y-direction as ry=Rleft∥Rright=Rleft*Rright/(Rleft+Rright). Let the resistance of the hypothetically isolated touch region 24 in the x-direction be Rx and the resistance in the y-direction be Ry. For example, if the touch region 24 has a uniform resistivity of ρ0, a width W and a height H, then Rx=ρ0*W/H and Ry=ρ0*H/W. The resistance ratios in the x- and y-directions are βx=rx/Rx and βy=ry/Ry. These resistance ratios β simultaneously provide a qualitative measure of power efficiency and a degree of non-linearity of the equipotential lines.
Specifically, if the resistance ratio β is very low (β<<1), nearly perfect equipotentials are impressed onto the touchscreen-—so good in fact that no topological or nonlinear mapping is required. However, it can also be mathematically proven that in the limit of small β,β is the power efficiency of the touchscreen, namely the fraction of power consumed by current flow in the touch region in contrast to power consumed in the band segments. Unless the substrate resistivity is very large (greater than a few thousand ohms/square), such low β designs lead to touchscreens of less than a few ohms resistance, drawing too much power. This is especially significant if the touchscreen is to be placed into hand-held, battery-powered devices, such as Personal Digital Assistants (PDAs). In general, for any given substrate resistivity, power consumption decreases with increasing resistance ratio β. If topological mapping is employed, however, the equipotential lines may be allowed to distort, thus allowing reduced power consumption. However, if the resistance ratio β becomes very high (β>>1), while the touchscreen power efficiency approaches 100%, the equipotentials tend to circle around the electrodes for both directions of excitation, as illustrated in FIG. 2a and 2b, thereby forming poor-crossing regions near the electrodes.
Even with elaborate topological mapping methodologies, the coordinates near the electrodes are difficult to determine uniquely. This problem could be addressed by increasing the effective border width of the touchscreen by simply masking off the perimeter region touch area. Excessive border width, however, is very undesirable from the point-of-view of compactness. Furthermore, some modern software applications with graphical user interfaces place much higher demands on touchscreen performance along the edges and corners of the touch region than was true ten or twenty years ago. Accuracy improvements in these corner regions are very desirable in order to reduce the maximum error on the touchscreen and to make its response more uniform.
There thus remains a need to improve the topological equivalence between the equipotential space and an external coordinate system near the electrodes of resistive touchscreens.